Superfund Research Program
Chloro-Organic Degradation by Polymer Membrane Immobilized Iron-Based Particle Systems
Project Leader: Dibakar Bhattacharyya
Grant Number: P42ES007380
Funding Period: 2000-2019
This project has continued to successfully demonstrate effective methods for the destruction of toxic, chlorinated organics (TCE, PCBs, CCl4) involving both oxidative (free-radical reaction pathways) and reductive (zero-valent nanoscale metals) dechlorination systems. With regard to membrane-based reductive dechlorination, Drs. Dibakar Bhattacharyya and Leonidas Bachas demonstrated the ability to prepare high metal (iron) loading membranes through in-situ polymerization of ion-exchange compounds (acrylic acid) inside membrane pores to capture metal ions as nanoparticle precursors, and to subsequently coat 2nd dopant metal (such as, Ni, Pd) on the iron nanoparticle surface. The particle size can be controlled through either manipulation of the spacing distance between chelated metal ions or by altering the metal loading. High resolution STEM-EDS mapping was used to localize the poly-acrylic acid (PAA) functionalization, characterize bimetallic nanoparticle structure, and correlate it with reactivity. Bimetallic Fe/Pd films have been used to effectively dechlorinate various chloro-organics including 3,3',4,4'-tetrachlorobiphenyl (PCB-77). For the first time the specialized computer modeling approach, along with the performed experiments, established the precise intrinsic reaction rates for PCB dechlorination. Complete dechlorination to less toxic biphenyl was observed even for low contact time. Palladium nanoparticles have also been formed electrochemically within conductive polypyrrole films and used for the successful reductive dechlorination of PCB-77 in the presence of H2; however, this system would require an external source of H2 in the absence of a corrosive metal, such as Fe0, that is normally used for H2 generation through its reaction with water.
The use of reductive pathways (such as bimetallic nanoparticles) leads to complete dechlorination without ring rupture (for example, PCB77 → biphenyl). Recent toxicity studies (in conjunction with the Superfund Chemicals, Nutrition, and Endothelial Cell Dysfunction Project) showed that PCB77 and biphenyl do not have the same toxic potency in primary vascular endothelial cells. Biphenyl appears to be non-toxic to the vascular endothelium. Such a trend would support the use of nanotechnology for reducing chlorinated organic pollutant levels in contaminated sites. Biphenyl is a hydrocarbon that could be bio-transformed to an unstable metabolite leading to possible toxicity through alternative pathways (DNA, or protein adducts, etc.). Further reduction of toxicity will also require ring cleavage by oxidation (modified Fenton Reaction) pathway.
Citrate and PAA (poly-chelate) based modified Fenton reaction has been used to effectively oxidize liquid TCE (DNAPL), 2,2’-PCB and biphenyl at neutral pH environments through the suppression of Fe2+ oxidation. More than 80% dechlorination of DNAPL was observed. The importance of superoxide radical formation was also verified by establishing direct dechlorination of carbon tetrachloride (CCl4). Finally, the ability to further protect the iron-chelate complex for potential treatment applications was demonstrated through immobilization using either silica particles or PVDF membranes functionalized with PAA.
Recently, collaborative work with the Kentucky Research Consortium for Energy and Environment (KRCEE) has been established to examine the potential use of both routes of dechlorination for the removal of trichloroethylene (TCE) at the Paducah Gaseous Diffusion Plant Superfund site in Paducah, KY. Preliminary testing examined the impact of matrix effects (background chemicals, oxygen, pH) on system performance. Both oxidative and nanotechnology-based treatments of TCE in a simulated groundwater column demonstrated greater than 50% TCE removal using minimal chemical dosing. One can adjust the chemical dosage (nanoparticles or oxidative treatment chemicals) to obtain the desired TCE removal criteria. The ability of chelate modified free radical reaction to dechlorinate DNAPL should also be very valuable for groundwater remediation.